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Shooting
Exothermic reactions and the fire triangle
Background
 Shooting was originally a functional activity used for hunting, to
provide food, and as an offensive and defensive method.
 Although still unfortunately often linked to acts of violence, shooting
is now a recognised sport and leisure activity when carried out in a
safe and controlled environment.
 The National Rifle Association (NRA),
founded in 1959, is the main governing body
of the sport in Britain and has its base at
Bisley, a world-renowned shooting venue.
Shooting and the link to Chemistry
Shooting involves the propulsion of a bullet from a barrel.
This propulsion can come from a variety of different
sources, which include:
 Stored kinetic energy from spring loaded guns.
 High air pressure in inside air guns, in which
compressed air propels a bullet out of the hamber
to release the build up of pressure.
 From a combustion reaction, such as when gunpowder
is ignited.
It is this final way in which a bullet can be fired that really links the
sport of shooting to chemistry, although all the methods have direct
links to science as a whole.
The fire triangle
In order for a fire to take place there must be three inter - relating
components present. These can be represented in a diagram known as the
fire triangle, which can be seen on the next slide.
 The first component which must be present is a fuel to burn. This is often,
but not always, carbon based.
 The second component which must be present is oxygen.
 The third component which must be present is a source of ignition (or
heat). This could be a spark or a flame for example. Once the fuel has
started to burn the energy released from the burning fuel will continue to
supply the energy needed for the burning to continue.
The fire triangle
Or ignition
source
When one or more of the three components is no longer present
the fire will stop.
Combustion reactions
Combustion reactions are reactions with oxygen that
are normally accompanied by a release of energy.
 Typically heat and light energy are given off during a
combustion reaction – a fire being a common example.
 If sound energy is also released the combustion
reaction is known as an explosion.
 When gunpowder is ignited a combustion reaction
(often in the form of a controlled explosion) takes place. It
is the release of energy from the reaction that will propel
a bullet from a gun, when gunpowder is used to fire
bullets.
Traditional gunpowder
Gunpowder, also known as black powder, is a granular mixture
of charcoal, sulfur and potassium nitrate, KNO3 (which is also
known as saltpetre).
The mixture, when ignited, is explosive and burns rapidly producing
volumes of gases and hot solids which can be used
as a propellant in firearms.
The composition of modern day gunpowder
is quite different to traditional gunpowder,
but the traditional powder composition was
developed as long ago as 1780.
Traditional gunpowder
The proportions, by weight, of the mixture of substances in traditional
gunpowder vary slightly depending on the purpose of the powder but
are normally roughly, 15% charcoal (softwood), 10% sulfur, and 75%
potassium nitrate. Each component has a specific purpose as
follows:
 The nitrate, typically potassium nitrate, supplies the oxygen for the
reactions which take place when the powder is ignited.
 The charcoal provides carbon and other fuel
sources for the reaction.
 The sulfur lowers the temperature of ignition
and increases the speed of combustion whilst also
acting as an additional fuel source.
Traditional gunpowder
The reactions which take place when gunpowder is ignited are extremely
complicated and difficult to represent in a chemical equation in full. However
the following is a widely accepted simple chemical equation which is
commonly used to represent the combustion of gunpowder:
2KNO3(s) + S(s) + 3C(s)
K2S(s) + N2(g) + 3CO2(g)
A still simplified equation that represents the reaction slightly more
accurately is:
10KNO3(s) + 3S(s) + 8C(s)
2K2CO3(s) + 3K2SO4(s) + 6CO2(g) + 5N2(g)
Exothermic and endothermic
reactions
When gunpowder is ignited energy is produced. More energy is produced
in the reaction than is needed for the reaction to take place. Chemists refer
to this type of reaction as being exothermic.
The chemical equation for an exothermic
reaction can be represented as follows:
Reactants
products + energy
Some reactions require more energy to take
place than is produced from the reaction.
These types of reaction are referred to as being
endothermic by chemists. They can be
represented as follows:
Reactants + energy
products
Enthalpy change, ∆H
 It is extremely difficult to measure the absolute amount of energy in a
chemical system. Chemists therefore measure the enthalpy change, ∆H.
 ∆H = energy used in bond breaking reactions – energy released in bond
making products.
 By definition the enthalpy change has a negative or minus value:
∆H < 0. In an exothermic reaction a negative ∆H is produced as a larger
value (the energy released in the reaction) is subtracted from a smaller value
(the energy used for the reaction) The opposite is true, i.e. a positive ∆H
value, for an endothermic reaction.
Enthalpy change, ∆H
Energy diagrams are often used to show if a reaction is endothermic or
exothermic. Below is an example of an energy diagram for an
exothermic reaction:
Energy diagram for the complete combustion of carbon.
C (s) + O2 (g)
∆H = - 393.5kJ
Increasing
energy
CO2 (g)
The information above tells a chemist that burning 12 g of carbon in
oxygen produces 393.5 kJ.
Enthalpy change, ∆H
An example of an energy diagram for an endothermic reaction can be found
below:
Energy diagram for the formation of hydrogen iodide.
2HI (g)
Increasing
energy
∆H = + 52 kJ
H2 (g) + 12 (g)
The information in this energy diagram tells a chemist that a net value of
+52 kJ of energy is required for the reaction to take place. As the
∆H is positive the reaction is endothermic.
Working out ∆H for a reaction from
data tables
Using data tables chemists can work out if a reaction will be exothermic or
endothermic. Here is an example using the formation of hydrogen chloride
from reacting hydrogen and chlorine together.
H2(g) + Cl2(g)
2HCl(g)
Using relative atomic masses, 2 g of hydrogen react with 71g of chlorine
to form 73 g of hydrogen chloride.
The data book states that it requires +436 kJ of energy to break the H-H
bonds in 2 g of hydrogen molecules and +242 kJ of energy to break the
Cl-Cl bonds in 71 g of chlorine molecules.
Working out ∆H for a reaction from
data tables
The data book also tells us that −431 kJ of energy are released when 36.5g of
hydrogen chloride is formed. As we form 2HCl from 2 g of H2 and 71 g of Cl2 then
73 g of HCl are formed. If it takes −431 kJ per 36.5g of HCl produced then the
energy required to produce 2HCL is twice as much, or −862 kJ.
The energy change is therefore:
+436 kJ (energy required to break H-H bonds in 2 g of hydrogen molecules) +
+242 kJ (energy required to break Cl-Cl bonds in 2 g of chlorine molecules) +
−862 kJ (energy released in the formation of 2 g of HCl) = −184 kJ.
Or: (+436 kJ) + (+242 kJ) + (−862 kJ) = −184 kJ
As the ∆H for the total reaction is negative then the reaction is exothermic.
1. What are the three inter-related components which
make up the fire triangle in order for fire to take
place? Explain the role of each component.
Source of fuel – required as something has to burn in order
for the fire to take place.
 Source of oxygen – required to allow the combustion
reaction causing the fire to take place.
 An initial ignition or source of heat to provide the energy for
the reaction to start.
2. What are the three most common components of
traditional gunpowder?
Charcoal, sulfur and potassium nitrate (often known as salt petre).
3. What is the difference between an exothermic
reaction and an endothermic reaction?
An exothermic reaction is a reaction whereby more energy is
produced and released from the reaction than is required in order for
the reaction to take place. An example of which is a combustion
reaction. An endothermic reaction requires more energy for the
reaction to take place than is created.
4. Using the following data, is the formation of water from
hydrogen and oxygen an exothermic or an endothermic
reaction?
Bond energies in kJ/mol: H-H is 436, O=O is 496 and O-H is
463. Show your workings.
The reaction is exothermic producing a net enthalpy change of +484 kJ
(242 kJ per mole of hydrogen burned or water formed).
Workings – Chemical equation is 2H2(g) + O2(g)
Bonds broken (energy taken in/needed):
(2 x H-H = 2 x 436) + (1 x O=O = 1 x 496)
= 1368 kJ
(answered continued on next slide)
2H2O(g)
Bonds made (energy produced):
(4 x O-H = 4 x 463)
= 1852 kJ
The overall energy change for the reaction is therefore:
−1368 kJ − −1852 kJ
= +484 kJ (or 242 kJ per mole of hydrogen burned or water formed).
As the enthalpy change is positive for the overall reaction the reaction
is exothermic.
5. Draw an energy diagram for the reaction in Q4.
Energy diagram for the formation of water from hydrogen and oxygen.
2H2 (g) + O2 (g)
∆H = - 484 kJ
Increasing
energy
2H2O2 (g)
The information in this energy diagram tells a chemist that a net value
−484 kJ of energy is required for the reaction to take place. As the ∆H is
negative the reaction is exothermic.